To reduce the fuel consumption and the pollutant emissions, different concepts are realized in modern motor vehicles which improve the degree of efficiency of the internal combustion engine, the exhaust gas purification or the aerodynamics of the motor vehicle. The implementation of a so-called automatic engine cut-off and starting system also belongs to these concepts, by means of which the internal combustion engine can be cut off and started again independently from the driver of the motor vehicle. The internal combustion engine is cut off here particularly during longer idling phases, in which the driving force of the internal combustion engine is not needed.
Notable reductions in fuel consumption can be achieved in this way in urban driving cycles for instance.
The cut-off of the internal combustion engine by the automatic engine cut-off and starting system is however subject to restrictions. For instance, the cut-off of the internal combustion engine can then be prevented if safety aspects (e.g. the maintenance of an adequate braking pressure) or a too minimal battery voltage require further operation of the internal combustion engine. It is also worthwhile for the internal combustion engine to reach an operating temperature as quickly as possible after start-up, whereby optimal conditions in respect of the power output, degree of efficiency and emission behavior prevail. In this respect it is known to prevent the cut-off of the internal combustion engine by means of the automatic engine cut-off and starting system until a certain operating temperature has been reached, so as not to slow down the heating process. The output signal of the coolant temperature sensor is knowingly used as a measure of the temperature of the internal combustion engine. The actual temperature of the internal combustion engine is only inadequately represented by the coolant temperature in the case of specific configurations and/or operating states. The internal combustion engine may have an auxiliary heating system, which heats the coolant to clearly higher temperatures, like the actual engine block. This method also assumes the presence of a fully functional coolant temperature sensor, so that this procedure cannot be used in the absence of or with a defect in the coolant temperature sensor, or may result in malfunctions.